1. Field of the Invention
The present invention relates to flat panel display technique, and particularly to a detecting circuit for pixel electrode voltage of a flat panel display.
2. Related Art
Liquid crystal display (LCD) devices have been widely used in various data processing equipments such as TV, notebook computer, mobile telephone, personal digital assistant, etc., due to the advantages of lighter weight, thinner thickness, smaller footprint, lower power consumption, lower radiation, and so on. With the continuous development of the electronic industry, the performance of the liquid crystal display devices is becoming better and better.
Taking the common thin film transistor LCD (TFT-LCD) as an example, it belongs to a type of active matrix liquid crystal display. The main feature of the TFT-LCD is that a semiconductor switching device is configured in each pixel point, and each pixel point is an independent transistor isolated from each other. Because each pixel point can be controlled directly by a point pulse, each pixel is relatively independent, and can be controlled continuously, which not only decreases the reaction time, but also can make it very accurate in gray scale control.
The ordinary liquid crystal device includes a liquid crystal display panel and a driving circuit for driving the liquid crystal display panel.
The liquid crystal display panel usually includes a color-film substrate and an array substrate. An array of M×N liquid crystal pixel units is arranged on a side of the array substrate that opposes to the color-film substrate. As shown in FIG. 1, taking any of the liquid crystal pixel units thereof as an example, it includes a scanning line GL, a data line DL crossing with the scanning line GL, and a thin film transistor (TFT) formed at the crossing of the scanning line GL and the data line DL to drive the liquid crystal pixel electrode. The scanning line GL is connected with the gate of the thin film transistor TFT to turn on the thin film transistor TFT; the data line DL is connected with the source of the thin film transistor TFT to supply voltage to the liquid crystal pixel electrode; and the liquid crystal pixel unit is connected with the drain of the thin film transistor. If a data voltage is applied to the pixel electrode that formed on the array substrate and a common electrode voltage Vcom is applied to the common electrode that located on the color-film substrate, the arrangement of the liquid crystal molecules is changed by the electric field that applied to the liquid crystal layer, thereby controlling the transmission amount of the ray, and displaying a corresponding image. In a pixel unit, a liquid crystal capacitor Clc is formed by the pixel electrode, the common electrode, and the liquid crystal molecules interposed therebetween, and the liquid crystal capacitor Clc is charged by the data line when the thin film transistor TFT is turned on, and makes the pixel electrode maintain the displaying voltage after the thin film transistor TFT is turned off until the thin film transistor is turned on next time. In view of the effect of the electric leakage of the liquid crystal capacitor Clc, a storage capacitor Cst is connected in parallel with the liquid crystal capacitor Clc. Moreover, a parasitic capacitor Cgs is provided between the gate terminal of the thin film transistor TFT connected with the scanning line GL and the source terminal of the thin film transistor TFT connected with the pixel electrode.
FIG. 2 shows a diagram of the electric potential variation of a voltage signal Vpixel of the liquid crystal pixel electrode with the variation of a gate driving pulse Vg and a data voltage Vdata. As shown in FIG. 2, a gate driving pulse Vg is applied to the scanning line GL to turn on the thin film transistor; when the gate driving pulse Vg is kept at a high gate voltage, i.e. the thin film transistor is turned on during the scanning period, a signal is applied to the pixel electrode by the data voltage Vdata, i.e. the liquid crystal capacitor Clc is charged, and a charging voltage may be kept for a constant time, the charging voltage also serving as the voltage for charging the storage capacitor Cst. It can be seen from FIG. 2 that, because of a kickback voltage caused by the parasitic capacitance of the thin film transistor, a voltage jump (labeled in circles in FIG. 2) occurs in the voltage signal Vpixel of the pixel electrode at the falling edge of the gate driving pulse, which in combination with the electric leakage of the thin film transistor switch and the image crosstalk may cause the voltage signal Vpixel of the pixel electrode to deviate from the given signal voltage. When the signal voltage Vpixel of each pixel electrode is not the voltage appropriate for the electronic image, the image contrast, the image flicker extent, the image residual extent, the image color saturation, the image gray scale, the GAMMA characteristic and the fidelity of the output optical image from the liquid crystal panel may distort.
Therefore, this type of asymmetric electric potential drift has to be calibrated by voltage compensation. Generally, the symmetry of the voltage signal of the pixel electrode can be achieved by adjusting the common electrode voltage Vcom to Vcom′. However, the approach of adjusting the common electrode voltage is applicable only to the case that the electric potential variations of all the liquid crystal pixel electrodes are the same. And in other cases, because the variations of the kickback voltages in different liquid crystal pixel electrodes are not the same, the approach of adjusting the common electrode voltage Vcom can not be adopted to compensate the voltage.
Nowadays, it has been proposed in the industry that the non-uniform kickback voltage may be compensated by balancing the resistance and the capacitance of the scanning line. A “liquid crystal panel having compensation capacitors for balancing RC delay effect” is provided in U.S. Pat. No. 6,842,200, in which the products of multiplication of resistance values and capacitance values of respective leads are made to be close to each other mainly by additionally providing a plurality of compensation capacitors (having predetermined capacitance value) connected to a plurality of leads respectively, so as to reduce the RC delay effect among the leads. However, the method mentioned above has the problems of design constraints and complex fabrication process, and it is difficult to acquire an exact capacitance value in the practical application.
In another aspect, if the deviation of the pixel electrode voltage is knowable, a compensation component with an opposite polarity can be applied to the common electrode. However, in the prior art, because of the lack of method for directly detecting the voltage of the pixel electrode, the voltage variation of the pixel electrode is substantially estimated indirectly from experience and computer simulation in the existing TFT array design and error detection correction, so that the voltage deviation of the signal is uniformly compensated for the product or a batch of products. However, this compensational adjustment has the following drawbacks: being complex in operation, and being time-consuming and energy-consuming; because the voltage variation of the pixel electrode is estimated indirectly from experience and the computer simulation, it can not be assured to obtain the exact voltage variation of the pixel electrode, which may affect the subsequent compensation result; and it is inconvenient to perform individual processing to compensate the aging and the drift of the product during usage in time for each product in the practical application. Moreover, if the pixel electrode voltage is inducted by an external induction circuit (such as an amplifier or an oscilloscope etc.), because the capacitance value of the induction circuit itself may apparently affect the liquid crystal pixel unit having a small capacitance value (0.1 pF to 1 pF), the variation of the pixel voltage can not be detected accurately as well.